Processes for producing acetic acid

ABSTRACT

Processes for the reduction and/or removal of permanganate reducing compounds (PRC&#39;S) formed by the carbonylation of methanol in the presence of a Group VIII metal carbonylation catalyst to produce acetic acid are disclosed. More specifically, processes for reducing and/or removing PRC&#39;s or their precursors from intermediate streams during the formation of acetic acid by said carbonylation processes are disclosed. In particular, processes in which a low boiling overhead vapor stream from a light ends column is subjected to a distillation to obtain an overhead that is subjected to an extraction to selectively remove and/or reduce PRC&#39;s from the process is disclosed. The processes include steps of recycling one or more return streams derived from the distillation step and/or the extraction step to a light ends column and/or a drying column in order to improve water control in the overall reaction system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional App. No.61/345,833, filed on May 18, 2010, the entirety of which is incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to processes for producing acetic acid and, inparticular, to improved processes for reducing and/or removingpermanganate reducing compounds formed during the carbonylation ofmethanol in the presence of a Group VIII metal carbonylation catalyst toproduce acetic acid.

BACKGROUND OF THE INVENTION

Among currently employed processes for synthesizing acetic acid, one ofthe most useful commercially is the catalyzed carbonylation of methanolwith carbon monoxide as taught in U.S. Pat. No. 3,769,329, incorporatedherein by reference in its entirety. The carbonylation catalyst containsrhodium, either dissolved or otherwise dispersed in a liquid reactionmedium or supported on an inert solid, along with a halogen-containingcatalyst promoter as exemplified by methyl iodide. The rhodium can beintroduced into the reaction system in any of many forms. Likewise,because the nature of the halide promoter is not generally critical, alarge number of suitable promoters, most of which are organic iodides,may be used. Most typically and usefully, the reaction is conducted bycontinuously bubbling carbon monoxide gas through a liquid reactionmedium in which the catalyst is dissolved.

An improvement in the prior art process for the carbonylation of analcohol to produce the carboxylic acid having one carbon atom more thanthe alcohol in the presence of a rhodium catalyst is disclosed in U.S.Pat. Nos. 5,001,259; 5,026,908; and 5,144,068; and EP0161874, theentireties of which are incorporated herein by reference. As disclosedtherein, acetic acid is produced from methanol in a reaction mediumcontaining methyl acetate (MeAc), methyl halide, especially methyliodide (MeI), and rhodium present in a catalytically effectiveconcentration. These patents disclose that catalyst stability and theproductivity of the carbonylation reactor can be maintained at highlevels, even at very low water concentrations, i.e., 4 weight percent orless, (despite the conventional practice of maintaining approximately14-15 wt. % water) by maintaining in the reaction medium, along with acatalytically effective amount of rhodium, at least a finiteconcentration of water, e.g., about 0.1 wt. %, and a specifiedconcentration of iodide ions over and above the iodide ion that ispresent as hydrogen iodide. This iodide ion is a simple salt, withlithium iodide being preferred. The patents teach that the concentrationof methyl acetate and iodide salts are significant parameters inaffecting the rate of carbonylation of methanol to produce acetic acid,especially at low reactor water concentrations. By using relatively highconcentrations of the methyl acetate and iodide salt, a high degree ofcatalyst stability and reactor productivity is achieved even when theliquid reaction medium contains water in finite concentrations as low asabout 0.1 wt. %. Furthermore, the reaction medium employed improves thestability of the rhodium catalyst, i.e., resistance to catalystprecipitation, especially during the product recovery steps of theprocess. In these steps, distillation for the purpose of recovering theacetic acid product tends to remove from the catalyst the carbonmonoxide, which in the environment maintained in the reaction vessel, isa ligand with stabilizing effect on the rhodium.

It has been found that although a low water carbonylation process forproducing acetic acid reduces such by-products as carbon dioxide,hydrogen, and propionic acid, the amount of other impurities, presentgenerally in trace amounts, can be increased by a low watercarbonylation process, and the quality of acetic acid sometimes sufferswhen attempts are made to increase the production rate by improvingcatalysts, or modifying reaction conditions.

These trace impurities affect quality of acetic acid, especially whenthey are recirculated through the reaction process, which, among otherthings, can result in the build up over time of these impurities. Theimpurities that decrease the permanganate time of the acetic acid, aquality test commonly used in the acetic acid industry, include carbonylcompounds and unsaturated carbonyl compounds. As used herein, the phrase“carbonyl” is intended to mean compounds that contain aldehyde or ketonefunctional groups, which compounds may or may not possess unsaturation.See Catalysis of Organic Reaction, 75, 369-380 (1998), for furtherdiscussion on impurities in a carbonylation process.

Carbonyl impurities, such as acetaldehyde, that are formed during thecarbonylation of methanol may react with iodide catalyst promoters toform multi-carbon alkyl iodides, e.g., ethyl iodide, propyl iodide,butyl iodide, pentyl iodide, hexyl iodide, and the like. It is desirableto remove multi-carbon alkyl iodides from the reaction product becauseeven small amounts of these impurities in the acetic acid product tendto poison the catalyst used in the production of vinyl acetate, aproduct commonly produced from acetic acid. Thus, the present inventionmay also lead to reduction or removal of multi-carbon alkyl iodides, inparticular C₂-C₁₂ alkyl iodide compounds. Accordingly, because manyimpurities originate with acetaldehyde, it is a primary objective toremove carbonyl impurities, notably acetaldehyde, from the process so asto reduce the multi-carbon alkyl iodide content.

Conventional techniques to remove such impurities include treating theacetic acid product streams with oxidizers, ozone, water, methanol,activated-carbon, amines, and the like. Such treatments may or may notbe combined with distillation of the acetic acid. The most typicalpurification treatment involves a series of distillations of the finalproduct. It is also known to remove carbonyl impurities from organicstreams by treating the organic streams with an amine compound such ashydroxylamine, which reacts with the carbonyl compounds to form oximes,followed by distillation to separate the purified organic product fromthe oxime reaction products. However, the additional treatment of thefinal product adds cost to the process, and distillation of the treatedacetic acid product can result in additional impurities being formed.

While it is possible to obtain acetic acid of relatively high purity,the acetic acid product formed by the low-water carbonylation processand purification treatment described above frequently remains somewhatdeficient with respect to the permanganate time due to the presence ofsmall proportions of residual impurities. Because a sufficientpermanganate time is an important commercial test, which the acidproduct may be required to meet to be suitable for many uses, thepresence of impurities that decrease permanganate time is objectionable.Moreover, it has not been economically or commercially feasible toremove minute quantities of these impurities from the acetic acid bydistillation because some of the impurities have boiling points close tothat of the acetic acid product or halogen-containing catalystpromoters, such as methyl iodide. It has thus become important toidentify economically viable methods of removing impurities elsewhere inthe carbonylation process without contaminating the final product oradding unnecessary costs.

The art has disclosed that carbonyl impurities present in the aceticacid product streams generally concentrate in the overhead from thelight ends column. Accordingly, the light ends column overhead has beentreated with an amine compound (such as hydroxylamine), which reactswith the carbonyl compounds to form derivatives that can be more easilyseparated from the remaining overhead by distillation, resulting in anacetic acid product with improved permanganate time.

It has been disclosed in U.S. Pat. Nos. 6,143,930 and 6,339,171 that itis possible to significantly reduce undesirable impurities in the aceticacid product by performing a multi-stage purification on the light endscolumn overhead. These patents disclose a purification process in whichthe light ends overhead is distilled twice, in each case taking theacetaldehyde overhead and returning a methyl iodide rich residuum to thereactor. The acetaldehyde-rich distillate obtained after the twodistillation steps is optionally extracted with water to remove themajority of the acetaldehyde for disposal, leaving a significantly loweracetaldehyde concentration in the raffinate that is recycled to thereactor. U.S. Pat. Nos. 6,143,930 and 6,339,171 are incorporated hereinby reference in their entirety.

In addition, it has been disclosed in US20060247466, the entirety ofwhich is incorporated herein by reference, that it is possible to reduceundesirable impurities in the acetic acid product by subjecting thelight ends overhead to a single distillation to obtain an overhead thatis subjected to an extraction to selectively remove and/or reducepermanganate reducing compounds (PRC's).

U.S. Pat. No. 7,223,886, the entirety of which is incorporated herein byreference, discloses a method for reducing the formation of alkyliodides and C₃₋₈ carboxylic acids by removing PRC's from the light phaseof the condensed light ends overhead stream, including (a) distillingthe light phase to yield a PRC enriched overhead stream; and (b)extracting the overhead stream with water in at least two consecutivestages and separating therefrom one or more aqueous streams containingPRC's.

While the above-described processes have been successful in reducingand/or removing carbonyl impurities from the carbonylation system andfor the most part controlling acetaldehyde levels and permanganate timeproblems in the final acetic acid product, further improvements canstill be made. Accordingly, there remains a need for alternativeprocesses to improve the efficiency of acetaldehyde removal. The presentinvention provides one such alternative solution.

SUMMARY OF THE INVENTION

This invention relates to processes for the production of acetic acid(HOAc) and, in particular, improved processes for the reduction and/orremoval of permanganate reducing compounds (PRC's) and alkyl iodidesformed by the carbonylation of methanol in the presence of a Group VIIImetal carbonylation catalyst to produce acetic acid. More specifically,this invention relates to improved processes for reducing and/orremoving PRC's or their precursors from intermediate streams during theformation of acetic acid by the carbonylation processes. The processesof the invention desirably minimize acetic acid recycle to the reactor,thereby reducing the load on the reactor and separation system,increasing production capability, debottlenecking the separation systemand decreasing utilities and the need for cooling water. In addition,the processes desirably provide an efficient means for controlling theamount of water in the reaction system, also resulting in reduced loadon the reactor and improved overall efficiency. An additional benefit ofthe processes of the invention is that they provide improved flexibilityin controlling the water load between the light ends column and thedrying column of the separation system.

In one embodiment, the invention is to a process for removing PRC's froma crude acetic acid composition comprising acetic acid and one or morePRC's, the process comprising the steps of: separating the crude aceticacid composition in a light ends column into a PRC enriched stream andan acetic acid stream; separating the PRC enriched stream into anaqueous phase and an organic phase; separating the aqueous phase in aPRC removal system (PRS) distillation column into an overhead stream anda bottoms stream; and directing a first return stream comprising analiquot portion of the bottoms stream to the light ends column. Theprocess may further comprise the step of combining the aliquot portionof the bottoms stream with one or more process streams to form the firstreturn stream. Preferably, the process further comprises the step ofextracting at least a portion of the overhead stream with an extractionmedium to form a PRC extracted stream and a raffinate, wherein the firstreturn stream further comprises at least a portion of the raffinate.Optionally, the process further comprises the steps of distilling theacetic acid stream in a drying column to form a water stream and anacetic acid product stream; and directing a second return streamcomprising an aliquot portion of the bottoms stream to the dryingcolumn. The second return stream optionally comprises the aliquotportion of the bottoms stream and a reflux stream. In one aspect, thesecond return stream further comprises an aliquot portion of theraffinate.

In one aspect, the process further comprises the steps of: distillingthe acetic acid stream in a drying column to form a water stream and anacetic acid product stream; and directing a second return streamcomprising an aliquot portion of the bottoms stream to the dryingcolumn. The second return stream optionally comprises the aliquotportion of the bottoms stream and a reflux stream. Optionally, theprocess further comprises the step of: extracting at least a portion ofthe overhead stream with an extraction medium to form a PRC extractedstream and a raffinate, wherein the second return stream furthercomprises a portion of the raffinate.

In another aspect, the process further comprises the steps of:extracting at least a portion of the overhead stream with an extractionmedium to form a PRC extracted stream and a raffinate, distilling theacetic acid stream in a drying column to form a water stream and anacetic acid product stream; and directing a second return streamcomprising an aliquot portion of the raffinate to the drying column.

In another embodiment, the invention is directed to a process forremoving PRC's from a crude acetic acid composition comprising aceticacid and one or more PRC's, the process comprising the steps of:separating the crude acetic acid composition in a light ends column intoa PRC enriched stream and an acetic acid stream; separating the PRCenriched stream into an aqueous phase and an organic phase; separatingthe aqueous phase in a PRS distillation column into an overhead streamand a bottoms stream; extracting at least a portion of the overheadstream with an extraction medium in an extraction unit to form a PRCextracted stream and a raffinate; and directing a first return streamcomprising an aliquot portion of the raffinate to the light ends column.The process optionally further comprises the step of combining thealiquot portion of the raffinate with one or more process streams toform the first return stream. Optionally, at least a portion of thebottoms stream is directed to the light ends column, optionally as partof the first return stream. In one aspect, the process further comprisesthe steps of: distilling the acetic acid stream in a drying column toform a water stream and an acetic acid product stream; and directing asecond return stream comprising a second aliquot portion of theraffinate to the drying column. In this aspect, at least a portion ofthe bottoms stream optionally is directed to the light ends column,optionally as part of the first return stream. In one aspect, at least aportion of the bottoms stream may be directed to the drying column,optionally as part of the second return stream. In another aspect, theprocess further comprises the steps of distilling the acetic acid streamin a drying column to form a water stream and an acetic acid productstream; and directing a second return stream comprising an aliquotportion of the bottoms stream to the drying column. The second returnstream may comprise the aliquot portion of the bottoms stream and areflux stream. Optionally, the first return stream further comprises analiquot portion of the bottoms stream.

In another embodiment, the invention is to a process for removing PRC'sfrom a crude acetic acid composition comprising acetic acid and one ormore PRC's, the process comprising the steps of: separating the crudeacetic acid composition in a light ends column into a PRC enrichedstream and an acetic acid stream; separating the PRC enriched streaminto an aqueous phase and an organic phase; separating the aqueous phasein a PRS distillation column into an overhead stream and a bottomsstream; distilling the acetic acid stream in a drying column to form awater stream and an acetic acid product stream; and directing a firstreturn stream comprising an aliquot portion of the bottoms stream to thedrying column. Optionally, the process further comprises the steps of:extracting at least a portion of the overhead stream with an extractionmedium to form a PRC extracted stream and a raffinate, wherein the firstreturn stream further comprises an aliquot portion of the raffinate. Theprocess may further comprise the steps of: extracting at least a portionof the overhead stream with an extraction medium to form a PRC extractedstream and a raffinate; and directing a second return stream comprisingan aliquot portion of the raffinate to the light ends column. In thisaspect, the first return stream may further comprise a portion of theraffinate.

In another embodiment, the invention is directed to a process for thereduction and/or removal of PRC's formed in the carbonylation of acarbonylatable reactant to produce a carbonylation product comprisingacetic acid, comprising the steps of: separating the carbonylationproduct in a first separation unit to form a first overhead streamcomprising acetic acid and a first bottoms stream comprising catalyst;distilling the first overhead stream in a second separation unit to forma crude acetic acid product and a second overhead stream comprisingmethyl iodide, water, acetic acid, methyl acetate, and at least one PRCyield; condensing the second overhead stream and biphasically separatingit to form a condensed heavy liquid phase and a condensed light liquidphase; distilling the condensed light liquid phase in a third separationunit to form a third overhead stream and a third bottoms stream, whereinthe third overhead stream is enriched with PRC's with respect to thecondensed light liquid phase; and directing a first return streamcomprising an aliquot portion of the third bottoms stream to the secondseparation unit. The process may further comprise the steps ofcondensing the third overhead stream and extracting the resultingcondensed stream with water to obtain an aqueous acetaldehyde streamcomprising PRC and a raffinate comprising methyl iodide, wherein thefirst return stream further comprises an aliquot portion of theraffinate, and optionally the steps of distilling the crude acetic acidproduct in a fourth separation unit to form a fourth overhead streamcomprising water and a fourth bottoms stream comprising acetic acidproduct; and directing a second return stream comprising an aliquotportion of the third bottoms stream to the fourth separation unit. Inone aspect, the process further comprises the steps of distilling thecrude acetic acid product in a fourth separation unit to form a fourthoverhead stream comprising water and a fourth bottoms stream comprisingacetic acid product; and directing a second return stream comprising analiquot portion of the third bottoms stream to the fourth separationunit.

In another embodiment, the invention is directed to a process for thereduction and/or removal of PRC's formed in the carbonylation of acarbonylatable reactant to produce a carbonylation product comprisingacetic acid, comprising the steps of: separating the carbonylationproduct in a first separation unit to form a first overhead streamcomprising acetic acid and a first bottoms stream comprising catalyst;distilling the first overhead stream in a second separation unit to forma crude acetic acid product and a second overhead stream comprisingmethyl iodide, water, acetic acid, methyl acetate, and at least one PRCyield; condensing the second overhead stream and biphasically separatingit to form a condensed heavy liquid phase and a condensed light liquidphase; distilling the condensed light liquid phase in a third separationunit to form a third overhead stream and a third bottoms stream, whereinthe third overhead stream is enriched with PRC's with respect to thecondensed light liquid phase; condensing the third overhead stream andextracting the resulting condensed stream with water to obtain anaqueous acetaldehyde stream comprising PRC and a raffinate comprisingmethyl iodide; and directing a first return stream comprising an aliquotportion of the raffinate to the second separation unit. The processoptionally further comprises the steps of distilling the crude aceticacid product in a fourth separation unit to form a fourth overheadstream comprising water and a fourth bottoms stream comprising aceticacid product; and directing a second return stream comprising an aliquotportion of the third bottoms stream to the fourth separation unit.

In another embodiment, the invention is to a process for the reductionand/or removal of PRC's formed in the carbonylation of a carbonylatablereactant to produce a carbonylation product comprising acetic acid,comprising the steps of: separating the carbonylation product in a firstseparation unit to form a first overhead stream comprising acetic acidand a first bottoms stream comprising catalyst; distilling the firstoverhead stream in a second separation unit to form a crude acetic acidproduct and a second overhead stream comprising methyl iodide, water,acetic acid, methyl acetate, and at least one PRC yield; condensing thesecond overhead stream and biphasically separating it to form acondensed heavy liquid phase and a condensed light liquid phase;distilling the condensed light liquid phase in a third separation unitto form a third overhead stream and a third bottoms stream, wherein thethird overhead stream is enriched with PRC's with respect to thecondensed light liquid phase; distilling the crude acetic acid productin a fourth separation unit to form a fourth overhead stream comprisingwater and a fourth bottoms stream comprising purified acetic acid; anddirecting a first return stream comprising an aliquot portion of thethird bottoms stream to the fourth separation unit. The processoptionally further comprises the steps of condensing the third overheadstream and extracting the resulting condensed stream with water toobtain an aqueous acetaldehyde stream comprising PRC and a raffinatecomprising methyl iodide, and directing a second return streamcomprising an aliquot portion of the raffinate to the second separationunit.

In another embodiment, the invention is directed to a process forremoving PRC's from a crude acetic acid composition comprising aceticacid and one or more PRC's, the process comprising the steps of:separating the crude acetic acid composition in a light ends column intoa PRC enriched stream and an acetic acid stream; separating at least aportion of the acetic acid stream in a drying column to form a waterstream and an acetic acid product stream; separating the PRC enrichedstream into an aqueous phase and an organic phase; separating theaqueous phase in a PRS distillation column into an overhead stream and abottoms stream; extracting at least a portion of the overhead streamwith an extraction medium to form a PRC extracted stream and araffinate; directing a first return stream comprising an aliquot portionof the bottoms stream to the light ends column; directing at least aportion of the raffinate to the light ends column, optionally as part ofthe first return stream; and directing a second return stream comprisingan aliquot portion of the bottoms stream to the drying column. Theprocess optionally further comprises the step of directing a portion ofthe raffinate to the drying column, optionally as part of the secondreturn stream.

For those embodiments that include an extraction step, the extractionmedium optionally comprises water and/or dimethyl ether. Preferably,dimethyl ether is present in the extracting step, whether separatelyadded or formed in situ, in an amount sufficient to reduce methyl iodideconcentration in the PRC extracted stream to an amount less than 1.8 wt.%, e.g., less than 1.5 wt. % or from 0.5 to 1.8 wt. %.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood in view of the appendednon-limiting figures, wherein:

FIG. 1 illustrates an exemplary acetic acid reaction system andseparation scheme according to one embodiment of the present invention;and

FIG. 2 illustrates an exemplary PRC removal system (PRS) according toone embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Introduction

This invention relates to processes for the production of acetic acidand, in particular, to improved processes for the reduction and/orremoval of permanganate reducing compounds (PRC's) formed during thecarbonylation of methanol in the presence of a Group VIII metalcarbonylation catalyst. PRC's, may include, for example, compounds suchas acetaldehyde (AcH), acetone, methyl ethyl ketone, butyraldehyde,crotonaldehyde, 2-ethyl crotonaldehyde, and 2-ethyl butyraldehyde andthe like, and the aldol condensation products thereof. Morespecifically, this invention relates to improved processes for reducingand/or removing PRC's or their precursors from an acetic acid separationsystem that is used to purify a crude acetic acid product.

In a preferred embodiment, the present invention relates to a process inwhich a crude acetic acid composition, preferably derived from acarbonylation reactor or flash vessel, is separated in a light endscolumn into a PRC enriched stream and an acetic acid stream, e.g., anacetic acid side stream. The PRC enriched stream is then separated intoan aqueous phase and an organic phase, preferably in a phase separationunit. The portion of the resulting aqueous phase is then directed to aPRC removal system (PRS) where it is separated in a PRS distillationcolumn into an overhead stream and a bottoms stream. The remainingportion of the aqueous stream may be refluxed to the light ends column.A return stream comprising an aliquot portion of the bottoms stream isthen returned to the light ends column, preferably to the top of thelight ends column, for further processing. In one embodiment, directinga portion or all of the return stream into a reflux of the light endscolumn. In one embodiment, a portion or all of the return stream may berefluxed to light ends column. In one embodiment, the bottoms stream isexclusively returned to the light ends column and no return stream isdirected from the PRS elsewhere in the system.

The PRS may further include a step of extracting at least a portion ofthe PRS distillation column overhead stream with an extraction medium toform a PRC extracted stream and a raffinate. In this aspect, the returnstream may further comprise at least a portion, e.g., an aliquotportion, or the entirety of the raffinate.

In another embodiment, the process includes the step of distilling anacetic acid stream derived from a light ends column in a drying columnto form a water stream and an acetic acid product stream. A secondreturn stream comprising an aliquot portion of the bottoms stream fromthe PRS distillation column is directed to the drying column, preferablyto the top of the drying column. In one embodiment, the bottoms streamreturned to the separation system is exclusively returned as the secondreturn stream to the drying column and no return stream is directed fromthe PRS to the light ends column. Additionally or alternatively, thesecond return stream may comprise an aliquot portion of the raffinatefrom the PRS extraction unit.

In one embodiment, the bottoms stream and optionally the raffinatestreams are directed either to the light ends column and/or dryingcolumn, but are not directed to the reactor or any recycle lines thatare fed to the reactor.

Among other advantages, the processes of the invention beneficiallyprovide efficient means for reducing and/or removing PRC's whileimproving water control in the overall acetic acid production system.This preferably results in smaller reactor and flash vessel sizeallowing for reduced capital expenditure. The processes of the inventionpreferably also reduce unnecessary acetic acid recycle to the reactor,thereby reducing the load on the separation system, resulting inincreased production capability. In addition, maintaining desired watercontrol is preferred to establish a stable acetic acid reaction system,and particularly for ensuring an efficient separation scheme. Additionaladvantages include, but are not limited to, lower energy usage andreduced equipment and associated costs.

Acetic Acid Production Systems

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

The purification processes of the present invention are useful incarbonylation processes that use methanol and/or methyl acetate (MeAc),methyl formate or dimethyl ether, or mixtures thereof, to produce aceticacid in the presence of a Group VIII metal catalyst, such as rhodium,and a halogen-containing catalyst promoter. A particularly usefulprocess is the low water rhodium-catalyzed carbonylation of methanol toacetic acid as exemplified in U.S. Pat. No. 5,001,259.

Generally, the rhodium component of the catalyst system is believed tobe present in the form of a coordination compound of rhodium with ahalogen component providing at least one of the ligands of suchcoordination compound. In addition to the coordination of rhodium andhalogen, it is also believed that carbon monoxide will coordinate withrhodium. The rhodium component of the catalyst system may be provided byintroducing into the reaction zone rhodium in the form of rhodium metal,rhodium salts such as the oxides, acetates, iodides, carbonates,hydroxides, chlorides, etc., or other compounds that result in theformation of a coordination compound of rhodium in the reactionenvironment.

The halogen-containing catalyst promoter of the catalyst system consistsof a halogen compound comprising an organic halide. Thus, alkyl, aryl,and substituted alkyl or aryl halides can be used. Preferably, thehalogen-containing catalyst promoter is present in the form of an alkylhalide. Even more preferably, the halogen-containing catalyst promoteris present in the form of an alkyl halide in which the alkyl radicalcorresponds to the alkyl radical of the feed alcohol, which is beingcarbonylated. Thus, in the carbonylation of methanol to acetic acid, thehalide promoter will include methyl halide, and more preferably methyliodide.

The liquid reaction medium employed may include any solvent compatiblewith the catalyst system and may include pure alcohols, or mixtures ofthe alcohol feedstock and/or the desired carboxylic acid and/or estersof these two compounds. A preferred solvent and liquid reaction mediumfor the low water carbonylation process contains the desired carboxylicacid product. Thus, in the carbonylation of methanol to acetic acid, apreferred solvent system contains acetic acid.

Water is contained in the reaction medium but desirably at lowconcentrations well below that which has heretofore been thoughtpractical for achieving sufficient reaction rates. It has previouslybeen taught, e.g., in U.S. Pat. No. 3,769,329, that in rhodium-catalyzedcarbonylation reactions of the type set forth in this invention, theaddition of water exerts a beneficial effect upon the reaction rate.Thus, commercial operations are commonly run at water concentrations ofat least about 14 wt. %. Accordingly, it has been quite unexpected thatreaction rates substantially equal to and above reaction rates obtainedwith such comparatively high levels of water concentration can beachieved with water concentrations below 14 wt. % and as low as about0.1 wt. %, e.g., from 0.1 wt. % to 14 wt. %, from 0.2 wt. % to 10 wt. %or from 0.25 wt. % to 5 wt. %, based on the total weight of the reactionmedium.

In accordance with the carbonylation process most useful to manufactureof acetic acid according to the present invention, the desired reactionrates are obtained even at low water concentrations by maintaining inthe reaction medium an ester of the desired carboxylic acid and analcohol, desirably the alcohol used in the carbonylation, and anadditional iodide ion that is over and above the iodide ion that ispresent as hydrogen iodide. A desired ester is methyl acetate. Theadditional iodide ion is desirably an iodide salt, with lithium iodide(LiI) being preferred. It has been found, as described in U.S. Pat. No.5,001,259, that under low water concentrations, methyl acetate andlithium iodide act as rate promoters only when relatively highconcentrations of each of these components are present and that thepromotion is higher when both of these components are presentsimultaneously. The concentration of iodide ion maintained in thereaction medium of the preferred carbonylation reaction system isbelieved to be quite high as compared with what little prior art thereis dealing with the use of halide salts in reaction systems of thissort. The absolute concentration of iodide ion content is not alimitation on the usefulness of the present invention.

The carbonylation reaction of methanol to acetic acid product may becarried out by contacting the methanol feed with gaseous carbon monoxidebubbled through an acetic acid solvent reaction medium containing therhodium catalyst, methyl iodide (MeI) promoter, methyl acetate (MeAc),and additional soluble iodide salt, at conditions of temperature andpressure suitable to form the carbonylation product. It will begenerally recognized that it is the concentration of iodide ion in thecatalyst system that is important and not the cation associated with theiodide, and that at a given molar concentration of iodide the nature ofthe cation is not as significant as the effect of the iodideconcentration. Any metal iodide salt, or any iodide salt of any organiccation, or other cations such as those based on amine or phosphinecompounds (optionally, ternary or quaternary cations), can be maintainedin the reaction medium provided that the salt is sufficiently soluble inthe reaction medium to provide the desired level of the iodide. When theiodide is a metal salt, preferably it is an iodide salt of a member ofthe group consisting of the metals of Group IA and Group IIA of theperiodic table as set forth in the “Handbook of Chemistry and Physics”published by CRC Press, Cleveland, Ohio, 2002-03 (83rd edition). Inparticular, alkali metal iodides are useful, with lithium iodide beingparticularly suitable. In the low water carbonylation process mostuseful in this invention, the additional iodide ion over and above theiodide ion present as hydrogen iodide is generally present in thecatalyst solution in amounts such that the total iodide ionconcentration is from about 2 to about 20 wt. % and the methyl acetateis generally present in amounts of from about 0.5 to about 30 wt. %, andthe methyl iodide is generally present in amounts of from about 5 toabout 20 wt. %. The rhodium catalyst is generally present in amounts offrom about 200 to about 2000 parts per million (ppm).

Typical reaction temperatures for carbonylation will be from about 150°C. to about 250° C., with the temperature range of about 180° C. toabout 220° C. being a preferred range. The carbon monoxide partialpressure in the reactor can vary widely but is typically from about 2 toabout 30 atmospheres, e.g., from about 3 to about 10 atmospheres.Because of the partial pressure of by-products and the vapor pressure ofthe contained liquids, the total reactor pressure will range from about15 to about 40 atmospheres.

An exemplary reaction and acetic acid recovery system that is used forthe iodide-promoted rhodium catalyzed carbonylation of methanol toacetic acid in accordance with one embodiment of the present inventionis shown in FIG. 1. As shown, a methanol-containing feed stream 1 and acarbon monoxide-containing feed stream 2 are directed to a liquid phasecarbonylation reactor 3, in which the carbonylation reaction occurs toform acetic acid.

The carbonylation reactor 3 is preferably either a stirred vessel orbubble-column type vessel, with or without an agitator, within which thereacting liquid or slurry contents are maintained, preferablyautomatically, a predetermined level, which preferably remainssubstantially constant during normal operation. Into reactor 3, freshmethanol, carbon monoxide, and sufficient water are continuouslyintroduced as needed to maintain at least a finite concentration ofwater, e.g., about 0.1 wt. %., in the reaction medium.

In a typical carbonylation process, carbon monoxide is continuouslyintroduced into the carbonylation reactor, desirably below the agitator,which may be used to stir the contents. The gaseous feed preferably isthoroughly dispersed through the reacting liquid by this stirring means.A gaseous purge stream 8 desirably is vented from the reactor 3 toprevent buildup of gaseous by-products and to maintain a set carbonmonoxide partial pressure at a given total reactor pressure. Thetemperature of the reactor may be controlled and the carbon monoxidefeed is introduced at a rate sufficient to maintain the desired totalreactor pressure.

The acetic acid production system preferably includes a separationsystem employed to recover the acetic acid and recycle catalystsolution, methyl iodide, methyl acetate, and other system componentswithin the process. Thus, a recycled catalyst solution, such as stream27 from flasher 5, and optionally one or more of recycle streams 6, 9,31, 41 and 48, also are introduced into the reactor 3. Of course, one ormore of the recycle streams may be combined prior to being introducedinto the reactor. The separation system also preferably controls waterand acetic acid content in the carbonylation reactor, as well asthroughout the system, and facilitates PRC removal.

Carbonylation product is drawn off from the carbonylation reactor 3 at arate sufficient to maintain a constant level therein and is provided toa flasher 5 via stream 4. In flasher 5, the crude product is separatedin a flash separation step to obtain a volatile (“vapor”) overheadstream 26 comprising acetic acid and a less volatile stream 27comprising a catalyst-containing solution (predominantly acetic acidcontaining the rhodium and the iodide salt along with lesser quantitiesof methyl acetate, methyl iodide, and water), which preferably isrecycled to the reactor, as discussed above. The vapor overhead stream26 also comprises methyl iodide, methyl acetate, water, and PRC's.Dissolved gases exiting the reactor and entering the flasher comprise aportion of the carbon monoxide and may also contain gaseous by-productssuch as methane, hydrogen, and carbon dioxide. Such dissolved gases exitthe flasher as part of the overhead stream. The overhead stream fromflasher 5 is directed to the light ends column 14 as stream 26, wheredistillation yields a low-boiling overhead vapor stream 28, a purifiedacetic acid product that preferably is removed via a side stream 17, anda high boiling residue stream 9. Acetic acid removed via side stream 17preferably is subjected to further purification, such as in dryingcolumn 43 for selective separation of acetic acid from water.

It has been disclosed in U.S. Pat. Nos. 6,143,930 and 6,339,171 thatthere is generally a higher concentration of the PRC's, and inparticular acetaldehyde content, in the low-boiling overhead vaporstream exiting the light ends column than in the high-boiling residuestream exiting the column. Thus, in accordance with the presentinvention, low-boiling overhead vapor stream 28, containing PRC's, issubjected to additional processing in a PRS 50 to reduce and/or removethe amount of PRC's present. As shown, low-boiling overhead vapor stream28, therefore, is condensed and directed to an overhead phase separationunit, as shown by overhead receiver decanter 16. In addition to PRC's,low-boiling overhead vapor stream 28 will typically contain methyliodide, methyl acetate, acetic acid, and water.

Conditions are desirably maintained in the process such that low-boilingoverhead vapor stream 28, once in decanter 16, will separate into alight phase and a heavy phase. Generally, low-boiling overhead vaporstream 28 is cooled to a temperature sufficient to condense and separatethe condensable methyl iodide, methyl acetate, acetaldehyde and othercarbonyl components, and water into two phases. A portion of stream 28may include noncondensable gases such as carbon monoxide, carbondioxide, hydrogen, and the like that can be vented as shown by stream 29in FIG. 1, which may be directed to a low pressure absorber unit (notshown).

The condensed light phase in decanter 16 generally will comprise water,acetic acid, and PRC's, as well as quantities of methyl iodide andmethyl acetate. The condensed heavy phase in decanter 16 will generallycomprise methyl iodide, methyl acetate, and PRC's. The condensed heavyliquid phase in the decanter 16 can be conveniently recirculated, eitherdirectly or indirectly, to the reactor 3 via stream 31. For example, aportion of this condensed heavy liquid phase can be recirculated to thereactor, with a slip stream (not shown), generally a small amount, e.g.,from 5 to 40 vol. %, or from 5 to 20 vol. %, of the heavy liquid phasebeing directed to a PRS. This slip stream of the heavy liquid phase maybe treated individually or may be combined with the condensed lightliquid phase stream 30 for further distillation and extraction ofcarbonyl impurities in accordance with one embodiment of the presentinvention.

Although the specific compositions of the light phase stream 30 may varywidely, some preferred compositions are provided below in Table 1.

TABLE 1 Exemplary Light Compositions from Light Ends Overhead conc. (Wt.%) conc. (Wt. %) conc. (Wt. %) HOAc 1-40 1-25 5-15 Water 50-90  50-80 60-80  PRC's  <5 <3 <1 MeI <10 <5 <3 MeAc 1-50 1-25 1-15

As shown in FIG. 1, the light phase exits decanter 16 via stream 30. Afirst portion, e.g., aliquot portion, of light phase stream 30 isrecycled to the top of the light ends column 14 as reflux stream 34. Asecond portion, e.g., aliquot portion, of light phase stream 30 isdirected to the PRS, as discussed below and as shown by stream 32. Athird portion, e.g., aliquot portion, of the light phase stream 30optionally may be recycled to reactor 3 as shown by recycle stream 6,when additional water is desired or needed in reactor 3. In preferredaspects the water level is maintained in the reactor at a desired levelwithout recycling stream 6 to reactor 3 since recycling stream 6 to thereactor undesirably will result in the recycle of acetic acid andunnecessarily increasing the load on reactor 3. Thus, the only recyclefrom decanter 16 to reactor 3 is through the heavy phase stream 31.

Light ends column 14 also preferably forms a residuum or bottoms stream9, which comprises primarily acetic acid and water. Since the light endsbottoms stream 9 typically will comprise some residual catalyst, it maybe beneficial to recycle all or a portion of the light ends bottomsstream 9 to reactor 3. Optionally, the light ends bottoms stream may becombined with the catalyst phase 27 from flasher 5 and returned togetherto reactor 3, as shown in FIG. 1.

As indicated above, in addition to the overhead phase, the light endscolumn 14 also forms an acetic acid side stream 17, which preferablycomprises primarily acetic acid and water. In order to maintain anefficient product separation, it is important that the composition ofthe side stream 17 does not vary or fluctuate significantly duringnormal operation. The inventive separation schemes of the presentinvention, which employ one or more return streams from the PRS, asdisclosed below, are highly effective in maintaining desirable watercontrol in the reaction system and, as a result, in maintaining asubstantially constant composition of side stream 17. In addition, theprocesses of the invention desirably provide the ability to controlwater content in the separation system, and in particular, provideimproved flexibility in controlling the water balance between the lightends column 14 and the drying column 43 resulting in improved reactionsystem stability.

Optionally, a portion of the side stream 17 may be recirculated to thelight ends column, preferably to a point below where side stream 17 wasremoved from light ends column, in order to improve the separation.

Since side stream 17 contains water in addition to acetic acid, sidestream 17 from the light ends column 14 preferably is directed to adrying column 43, in which the acetic acid and water are separated fromone another. As shown, drying column 43, separates acetic acid sidestream 17 into an overhead stream 44 comprised primarily of water and abottoms stream 45 comprised primarily of acetic acid. Overhead stream 44preferably is cooled and condensed in a phase separation unit, e.g.,decanter 46, to form a light phase and a heavy phase. As shown, aportion of the light phase is refluxed, as shown by streams 47 and 53and the remainder of the light phase is returned to the reactor 3, asshown by stream 41. The heavy phase, which typically is an emulsioncomprising water and methyl iodide, preferably is returned in itsentirety to the reactor 3, as shown by stream 48, optionally after beingcombined with stream 41. Exemplary compositions for the light phase ofthe drying column overhead are provided below in Table 2.

TABLE 2 Exemplary Light Compositions from Drying Column Overhead conc.(Wt. %) conc. (Wt. %) conc. (Wt. %) HOAc 1-20 1-15 1-10 Water 50-90 60-90  70-90  MeI <10 <5 <3 MeAc 1-20 1-15 1-10

The drying column bottoms stream 45 preferably comprises or consistsessentially of acetic acid. In preferred embodiments, the drying columnbottoms stream comprises acetic acid in an amount greater than 90 wt. %,e.g., greater than 95 wt. % or greater than 98 wt. %. Optionally, thedrying column bottoms stream 45 may be further processed, e.g. bypassing through an ion exchange resin, prior to being stored ortransported for commercial use.

PRC Removal System (PRS)

The present invention may broadly be considered as an improved processfor reducing and/or removing PRC's, primarily aldehydes such asacetaldehyde, from a low-boiling overhead vapor stream of a light endsdistillation column, more preferably from the condensed light phase of alow-boiling overhead vapor stream 28 from a light ends distillationcolumn 14. Specifically, the invention relates to recycling one or morereturn streams from a PRS to the separation system of an acetic acidproduction system. Thus, no return streams from the PRS 50 are directedto the reactor 3 or to recycle lines to reactor 3. The PRS preferablyincludes at least one distillation column and at least one extractioncolumn to reduce and/or remove PRC's. Although the invention isdescribed herein with reference to FIG. 1 in terms of a PRS having asingle distillation column, it should be understood that it may besimilarly adapted to a multi-distillation PRC removal system, asdescribed in U.S. Pat. No. 6,339,171. For example, as shown in FIG. 2,if two PRC distillation columns are employed, 81, 82, the bottomsstreams 83, 84, from both distillation columns, separately or incombination, may be returned to either or both the light ends columnand/or the drying column, as shown in exemplary PRS 80. Overhead stream85 from first PRC distillation column 81 preferable is directed tosecond PRC distillation column 82, and the overhead from the second PRCdistillation column preferably is extracted, as discussed above inconnection with FIG. 1.

Similarly, although the PRS described and shown in FIG. 1 contains asingle extraction step, it should be understood that the system mayinclude multiple extraction stages, as described for example in U.S.Pat. No. 7,223,886 and optionally including multistage countercurrentextraction. According to various embodiments of the invention, one ormore streams derived from either or both (i) the PRS distillation columnand/or (ii) the PRS extraction stage, for example, may be returned toeither or both (i) the light ends removal column and/or (ii) the dryingcolumn of the separation system for the acetic acid production system.For purposes of the present specification and claims, the overheadstreams and overhead decanters of the light ends removal column and thedrying column are considered to be part of the light ends removal columnand of the drying column.

As indicated above, while either phase of the low-boiling overhead vaporstream 28, may be subsequently processed to remove the PRC's, andprimarily the acetaldehyde component of the stream, in the presentinvention, the PRC's are removed from a portion, e.g., an aliquotportion, of the condensed light liquid phase 30, as shown by stream 32,which is directed to the PRS 50.

In accordance with the present invention, stream 32 is directed to adistillation column 18 in the PRS, which serves to form an overheadstream 36 enriched in PRC's, notably acetaldehyde, with respect to thelight condensed liquid phase 30/32, but also containing methyl iodidedue to the similar boiling points of methyl iodide and acetaldehyde. Theoverhead stream 36 is deficient of methyl acetate, methanol, and/oracetic acid (desirably all three) with respect to light condensed liquidphase 30/32 and with respect to bottoms stream 38, which is also formedby distillation column 18. Bottoms stream 38 preferably is enriched inwater, acetic acid, methanol, and/or methyl acetate (desirably all four)when compared with the composition of any one of streams 30, 32 and 36.

In one embodiment, a portion, e.g., an aliquot portion, of bottomsstream 38 from PRS distillation column 18 is refluxed to thedistillation column 18, as shown by stream 39. The reflux ratio ofstream 39 to the remaining bottoms stream optionally is from 0.005:1 to0.5:1, e.g., from 0.01:1 to 0.1:1. Although the compositions may varydepending on the specific PRS employed, some exemplary compositions ofthe PRS distillation column overhead stream 36 and bottoms stream 38 areprovided in Table 3.

TABLE 3 Exemplary Overhead and Bottoms Stream Compositions for PRSDistillation Column Overhead 36 Bottoms 38 conc. conc. conc. conc. conc.conc. (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) HOAc  <3 <1 <0.51-40 1-30 1-20 water <15 <10  <5 50-90  60-90  70-90  PRC's 10-40 15-4020-40 <0.3 <0.2 <0.1 MeI 50-90 60-90 60-80 <10 <5 <3 MeAc <10 <5 <2 5-505-25 5-15

In one optional embodiment, not shown, PRS distillation column 18 alsoforms a side stream enriched in methyl acetate, which might otherwisebuild up in the center of column 18 or be pushed into the overheadstream 36. This side stream allows the distillation column 18 to beoperated under conditions desirable for obtaining a higher concentrationof acetaldehyde in overhead stream 36 while providing a mechanism forremoving methyl acetate. The side stream, comprising methyl acetate,preferably is retained in the process, and may be returned to thereactor.

In a first aspect of the invention, a first portion, e.g., an aliquotportion, of bottoms stream 38 is directed to light ends column 14 as areturn stream 75 for further processing in the separation system.Optionally, the first portion of bottoms stream 38 is combined with oneor more other streams, such as raffinate 74, prior to being introducedinto light ends column 14 as shown in FIG. 1 and as described in greaterdetail below.

In a second aspect of the invention, which optionally may be combinedwith the above described first aspect of the invention, a second portion54, e.g., an aliquot portion, of the bottoms stream 38 is directed todrying column 43, preferably the upper portion of drying column 43, as areturn stream for further processing. The second portion 54 of bottomsstream 38 optionally may be combined with one or more other streamsprior to being introduced into drying column 43. As shown in FIG. 1, thesecond portion 54 may be (i) added directly to the top of drying column43 as shown by stream 51, (ii) added as stream 52 to a light phasereflux derived from drying column decanter 46 to form a combined stream53 that is introduced into the drying column 43; or (iii) both. Inadditional optional embodiments, all or a portion of second portion 54may be added directly to decanter 46 and/or be returned to reactor 3.

In embodiments that include both the first and the second aspects, thevolume ratio of the amount of the PRS distillation column bottomsstreams that is directed to the light ends column to the amount of thebottoms stream that is directed to the drying column may vary widely,and depends on issues such as water control, the size of the columns,the catalyst employed, the presence of impurities, and feed rate to thePRS. In a preferred aspect, a volume ratio is greater than 1:1, e.g.,greater than 4:1, greater than 8:1, or greater than 10:1. In terms ofranges, the volume ratio of the amount of the PRS distillation columnbottoms streams that is directed to the to the light ends column to theamount of the bottoms stream that is directed to the drying columnranges from 1:1 to 20:1, e.g., from 5:1 to 15:1, from 9:1 to 13:1 orabout 11:1.

For purposes of the present specification, it should be understood thatthe term “aliquot portion” refers to both: (i) a portion of a parentstream that has the same composition as the parent stream from which itis derived, and (ii) a stream comprising a portion of a parent streamthat has the same composition as the parent stream from which it isderived and one or more additional streams that have been combinedtherewith. Thus, directing a return stream comprising an aliquot portion75 of the PRS distillation bottoms stream 38 to the light ends column 14encompasses the direct transfer of a portion of the PRS distillationbottoms stream to the light ends column as well as the transfer of aderivative stream comprising (i) a portion of the PRS distillationbottoms stream and (ii) one or more additional streams that are combinedtherewith prior to introduction into the light ends column. An “aliquotportion” would not include, for example, streams formed in adistillation step or a phase separation step, which would not becompositionally the same as the parent stream from which they arederived nor derived from such a stream.

The vapor phase 36 from PRS distillation column 18 is cooled andcondensed in receiver 20, e.g., a knock out drum. At least a portion,e.g., an aliquot portion, of resulting condensed stream 40 is directedto an extraction unit 70, in which it is extracted with an extractionagent, e.g., water, to reduce and/or remove PRC's, notably acetaldehyde.In a preferred embodiment, a portion of condensed stream 40 is refluxedas stream 42 to distillation column 18, optionally after being combinedwith a portion 39 of PRS distillation column bottoms stream 38 as shownin FIG. 1.

In extraction unit 70, acetaldehyde is extracted from stream 40 with theextraction agent 71, e.g., water, to reduce PRC's, notably acetaldehyde,and to obtain an extracted stream 72 comprising water, acetaldehyde, andmethyl iodide. The amount of methyl iodide may vary depending on theextraction agent, extraction conditions and composition of stream 40.Thus, acetaldehyde is reduced and/or removed from the separationprocess. Extracted stream 72 will generally be treated as a waste,although in some embodiments acetaldehyde may be stripped, with thewater being recirculated to the process, such as for the water used inextraction unit 70. The efficiency of the extraction will depend on suchthings as the number of extraction stages and the water to feed ratio.

Extraction with water can be either a single stage or multi stageextraction and any equipment used to conduct such extractions can beused in the practice of the present invention. Although a single stageextraction is shown in FIG. 1, multistage extraction, optionallycountercurrent multistage extraction, is preferred. For example,extraction can be accomplished by combining stream 40 with water 71 andproviding the combination successively to a mixer and then a separator.A suitable multiple extraction unit is described in U.S. Pat. No.7,223,886, the entire contents and disclosure of which is herebyincorporated by reference. Combinations of mixer(s) and separator(s) canbe operated in series to obtain a multistage extraction. Multistageextraction is desirably accomplished in a single vessel having a seriesof trays or other internals designed to facilitate efficientliquid-liquid contacting between the two phases. The vessel may beequipped with paddle(s) or other mechanisms for agitation to increaseextraction efficiency. In such a multistage extraction vessel, stream 40desirably is provided proximate to one end of the vessel with waterbeing provided proximate to the other end of the vessel or such otherlocation to obtain a countercurrent flow.

The mutual solubility between the two phases in the extraction canincrease with temperature. Accordingly, it is desirable that theextraction be conducted at a combination of temperature and pressuresuch that the extractor contents can be maintained in the liquid state.Moreover, it is desirable to minimize the temperatures to which stream40 is exposed to minimize the likelihood of polymerization andcondensation reactions involving acetaldehyde. Water 71 used in theextraction unit 70 is desirably from an internal stream so as tomaintain water balance within the reaction system. Dimethyl ether (DME)can be introduced to the extraction to improve the separation of methyliodide in the extraction, i.e., to reduce the loss of methyl iodide intothe extracted stream 72. Additionally or alternatively, the processconditions or PRS system may be designed such that the DME may be formedin situ. For example, in one embodiment, DME may be present in theextraction step, whether separately added or formed in situ, in anamount sufficient to reduce the methyl iodide concentration in extractedstream 72 to an amount less than 1.8 wt. %, e.g., less than 1.5 wt. %,less than 1.0 wt. % or less than 0.7 wt. %. In terms of ranges, DME maybe present in the extraction step, whether separately added or formed insitu, in an amount sufficient to reduce the methyl iodide concentrationin extracted stream 72 to an amount from 0.5 to 1.8 wt. %, e.g., from0.5 to 1.5 wt. %, from 0.5 to 1.0 wt. %, or from 0.5 to 0.7 wt. %.

According to a preferred third aspect of the invention, which may becombined with either or both the first and second aspects of theinvention, the raffinate 74 from the extraction, notably containingmethyl iodide, desirably is returned to light ends column 14, optionallyafter being combined with one or more other streams, such as firstportion 75, as shown. Once introduced into the light ends column, themethyl iodide from the raffinate 74 preferably exits the light endscolumn primarily through the overhead stream 28 and will accumulate inthe heavy phase of the corresponding phase separation unit 16.

According to a fourth aspect of the invention, which may be combinedwith one or more of the first, second and/or third aspects of theinvention, all or a portion of raffinate 74 from the extraction isreturned to drying column 43, optionally after being combined with oneor more other streams, such as second portion 54, as shown by dashedarrow 55. Once introduced into the drying column, the methyl iodide fromthe raffinate 74 preferably exits the drying column primarily throughthe overhead stream 44 and will accumulate in the heavy phase of thecorresponding phase separation unit 46. Methyl acetate from theraffinate 74 preferably follows the same route.

In another optional embodiment, not shown, all or a portion of raffinate74 may be added directly to decanter 16 and/or may be returned toreactor 3.

In the non-limiting embodiment shown, the raffinate in stream 74 iscombined with an aliquot portion 75 of PRS distillation column bottomsstream 38 to form combined stream 76. Combined stream 76 is then addedto light ends column reflux stream 34 and the resulting stream is addedto the top of light ends column 14. Of course, as will be appreciated bythose skilled in the art, these streams may be added to light endscolumn 14 in a variety of other configurations. For example, the streamsmay be combined in a different order or added separately to the lightends column 14.

In the present invention, the efficiency of separating acetaldehyde frommethyl iodide is primarily affected by the relative solubility ofacetaldehyde and methyl iodide in water. While acetaldehyde is misciblein water, methyl iodide is not. However, the solubility of methyl iodidein water increases, with a concomitant loss of methyl iodide from theprocess system, with increasing levels of methyl acetate and/ormethanol. At high enough methyl acetate and/or methanol levels, phaseseparation of methyl iodide in the water extraction may not occur.Similarly, phase separation of methyl iodide in the water extraction maynot occur if acetic acid concentrations are sufficiently high. Thus, itis desirable that the distillate that is condensed and provided forextraction, e.g., as stream 40, contains methanol and methyl acetate ata combined concentration of less than about 10 wt. %, more desirablyless than about 5 wt. %, even more desirably less than about 2 wt. %,and even more desirably less than about 1.5 wt. %. It is desirable thatthe distillate that is condensed and provided for extraction containless than about 3 wt. % acetic acid, more desirably less than about 1wt. %, and even more desirably less than about 0.5 wt. %. Particularlydesired would be acetic acid concentrations approaching 0 wt. %.

Thus, in the process of the present invention, a distillation,optionally a single distillation, is conducted in distillation column 18under conditions designed to control, notably minimize, the quantitiesof methyl acetate and acetic acid in vapor phase 36. Desirably,minimization of quantities of methyl acetate and acetic acid in vaporphase 36 is achieved while simultaneously maintaining higheracetaldehyde levels in vapor phase 36 than in the residuum ofdistillation column 18. It is desirable that the residuum ofdistillation column 18 contain less than about 0.3 wt. % acetaldehyde,more desirably less than about 0.2 wt. %, and even more desirably lessthan about 0.1 wt. %. Particularly desired would be acetaldehydeconcentrations approaching zero wt. %.

Thus, in accordance with one embodiment of the present invention,illustrated in FIG. 1, low-boiling overhead vapor stream 28 is condensedin overhead receiver decanter 16 where it is biphasically separated toform a condensed heavy liquid phase 31 and a light condensed liquidphase 30. The light condensed liquid phase 30 is provided to PRSdistillation column 18 via streams 30/32. In this and other embodimentsof the present invention, a portion of stream 30 can be directed back tothe light ends column 14 as reflux stream 34, optionally after havingbeen combined with either or both a portion 75, e.g., an aliquotportion, of PRS distillation column bottoms stream 38 and/or raffinatestream 74. Preferably a combined stream 76 is combined with refluxstream 34.

One of ordinary skill in the art having the benefit of this disclosurecan design and operate a PRS distillation column to achieve the desiredresults of the present invention. Such efforts, although possiblytime-consuming and complex, would nevertheless be routine for one ofordinary skill in the art having the benefit of this disclosure.Accordingly, the practice of this invention is not necessarily limitedto specific characteristic of a particular distillation column or theoperation characteristics thereof, such as the total number of stages,the feed point, reflux ratio, feed temperature, reflux temperature,column temperature profile, and the like.

EXAMPLE

The benefits of reduced acid recycle to the reactor according to onenon-limiting embodiment of the present invention will be betterunderstood in view of the following mass balance example.

When operating at low water composition, the methanol carbonylation unitdoes not consume or produce water. The reactor water composition iscontrolled by manipulating the flow rate of the recycle streams from thepurification section back to the reactor. Therefore, at steady state,the total water contained in the recycle streams will be determined bythe desired set point for reactor water composition. Using ASPEN Plus7.1 simulation software, a comparison between a conventional process andan improved process according to the embodiment of the invention shownin FIG. 1 was determined by mass balance calculations such that thetotal amount of water contained in the recycle streams was the same inboth cases. In this mass balance example, as shown in FIG. 1, onealiquot portion of the bottoms stream from the PRC distillation columnwas directed to the light ends column and second portion 54 of thebottoms stream from the PRC distillation column was directed to thedrying column 43. The raffinate 74 from the extraction unit 70 was alsodirected to the light ends column 14.

With this assumption, the compositions of water recycle streams 41 andsecond portion 54 of FIG. 1 according to one embodiment of the inventionwere determined by mass balance and compared with streams from aconventional process where the light phase in the light ends decanter isrecycled to the reactor (Stream 6). The results are shown in Table 4.

TABLE 4 Flow Water Acetic Acid (tons/hr) (Wt. %) (Wt. %) ConventionalProcess Stream 6 3.0 70 15 Stream 41 26 80  3 Stream 54 0 — — Total AcidRecycle 1.23 Inventive Process Stream 6 0 — — Stream 41 28.6 80  3Stream 54 2.8 75 20 Total Acid Recycle 0.858

As shown, the net result of the improved process is a decrease in therecycle rate of light phase from the light ends column and an increasein the recycle rate of the light phase from the drying column. Since thelight phase of the light ends column contains a higher percentage ofacetic acid than the light phase of the drying column, the improvedprocess surprisingly and unexpectedly has 30% less acetic acid recyclethan the conventional process when recycling the same total amount ofwater. This reduction in recycle of acetic acid is a significant benefitof the process shown in FIG. 1 and described herein over conventionalprocesses.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference. In addition, it should beunderstood that aspects of the invention and portions of variousembodiments and various features recited below and/or in the appendedclaims may be combined or interchanged either in whole or in part. Inthe foregoing descriptions of the various embodiments, those embodimentswhich refer to another embodiment may be appropriately combined withother embodiments as will be appreciated by one of skill in the art.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention.

We claim:
 1. A process for removing permanganate reducing compounds(PRC's) from a crude acetic acid composition comprising acetic acid andone or more PRC's, the process comprising the steps of: (a) separatingthe crude acetic acid composition in a light ends column into a PRCenriched stream and an acetic acid stream; (b) separating the PRCenriched stream into an aqueous phase and an organic phase; (c)separating the aqueous phase in a distillation column into an overheadstream and a bottoms stream; and (d) directing a first return streamcomprising an aliquot portion of the bottoms stream to the light endscolumn.
 2. The process of claim 1, further comprising the step ofcombining the aliquot portion of the bottoms stream with one or moreprocess streams to form the first return stream.
 3. The process of claim1, wherein the aqueous phase is separated in one or more distillationcolumns
 4. The process of claim 1, further comprising directing thefirst return stream into a reflux of the light ends column.
 5. Theprocess of claim 1, further refluxing the first return stream to thelight ends column
 6. The process of claim 1, further comprising the stepof: (e) extracting at least a portion of the overhead stream with anextraction medium to form a PRC extracted stream and a raffinate,wherein the first return stream further comprises at least a portion ofthe raffinate.
 7. The process of claim 6, wherein the extraction mediumis selected from the group consisting of water, dimethyl ether, andmixtures thereof.
 8. The process of claim 7, wherein dimethyl ether ispresent in the extracting step in an amount sufficient to reduce methyliodide concentration in the PRC extracted stream to an amount less than1.8 wt.%.
 9. The process of claim 7, wherein dimethyl ether is presentin the extracting step in an amount sufficient to reduce methyl iodideconcentration in the PRC extracted stream to an amount less than 1.0wt.%.
 10. The process of claim 7, wherein dimethyl ether is present inthe extracting step in an amount sufficient to reduce methyl iodideconcentration in the PRC extracted stream to an amount from 0.5 to 1.8wt.%.
 11. The process of claim 7, further comprising the steps of: (f)distilling the acetic acid stream in a drying column to form a waterstream and an acetic acid product stream; and (g) directing a secondreturn stream comprising an aliquot portion of the bottoms stream to thedrying column.
 12. The process of claim 11, wherein the second returnstream further comprises an aliquot portion of the raffinate, a refluxstream of the drying column, or a combination thereof.
 13. The processof claim 1, further comprising the steps of: (e) distilling the aceticacid stream in a drying column to form a water stream and an acetic acidproduct stream; and (f) directing a second return stream comprising analiquot portion of the bottoms stream to the drying column.
 14. Theprocess of claim 13, further comprising the step of: (g) extracting atleast a portion of the overhead stream with an extraction medium to forma PRC extracted stream and a raffinate, wherein the second return streamfurther comprises a portion of the raffinate.
 15. The process of claim1, further comprising the steps of: (e) extracting at least a portion ofthe overhead stream with an extraction medium to form a PRC extractedstream and a raffinate; (f) distilling the acetic acid stream in adrying column to form a water stream and an acetic acid product stream;and (g) directing a second return stream comprising an aliquot portionof the raffinate to the drying column.
 16. A process for removingpermanganate reducing compounds (PRC's) from a crude acetic acidcomposition comprising acetic acid and one or more PRC's, the processcomprising the steps of: (a) separating the crude acetic acidcomposition in a light ends column into a PRC enriched stream and anacetic acid stream; (b) separating the PRC enriched stream into anaqueous phase and an organic phase; (c) separating the aqueous phase ina distillation column into an overhead stream and a bottoms stream; (d)extracting at least a portion of the overhead stream with an extractionmedium in an extraction unit to form a PRC extracted stream and araffinate; and (e) directing a first return stream comprising an aliquotportion of the raffinate to the light ends column.
 17. The process ofclaim 16, wherein the extraction medium is selected from the groupconsisting of water, dimethyl ether, and mixtures thereof.
 18. Theprocess of claim 16, further comprising the step of combining thealiquot portion of the raffinate with one or more process streams toform the first return stream.
 19. The process of claim 16, wherein atleast a portion of the bottoms stream is directed to the light endscolumn, optionally as part of the first return stream.
 20. The processof claim 16, further comprising the steps of: (f) distilling the aceticacid stream in a drying column to form a water stream and an acetic acidproduct stream; and (g) directing a second return stream comprising analiquot portion of the bottoms stream to the drying column.
 21. Theprocess of claim 20, wherein the second return stream comprises thealiquot portion of the bottoms stream and a reflux stream.
 22. Theprocess of claim 16, wherein the first return stream further comprisesan aliquot portion of the bottoms stream.
 23. A process for removingpermanganate reducing compounds (PRC's) from a crude acetic acidcomposition comprising acetic acid and one or more PRC's, the processcomprising the steps of: (a) separating the crude acetic acidcomposition in a light ends column into a PRC enriched stream and anacetic acid stream; (b) separating the PRC enriched stream into anaqueous phase and an organic phase; (c) separating the aqueous phase ina distillation column into an overhead stream and a bottoms stream; (d)distilling the acetic acid stream in a drying column to form a waterstream and an acetic acid product stream; and (e) directing a firstreturn stream comprising an aliquot portion of the bottoms stream to thedrying column.
 24. The process of claim 23, wherein the first returnstream comprises the aliquot portion of the bottoms stream and a refluxstream.
 25. The process of claim 23, further comprising the step of: (f)extracting at least a portion of the overhead stream with an extractionmedium to form a PRC extracted stream and a raffinate, wherein the firstreturn stream further comprises an aliquot portion of the raffinate. 26.The process of claim 25, wherein the extraction medium is selected fromthe group consisting of water, dimethyl ether, and mixtures thereof. 27.The process of claim 23, further comprising the steps of: (f) extractingat least a portion of the overhead stream with an extraction medium toform a PRC extracted stream and a raffinate; and (g) directing a secondreturn stream comprising an aliquot portion of the raffinate to thelight ends column.
 28. A process for removing permanganate reducingcompounds (PRC's) from a crude acetic acid composition comprising aceticacid and one or more PRC's, the process comprising the steps of: (a)separating the crude acetic acid composition in a light ends column intoa PRC enriched stream and an acetic acid stream; (b) separating at leasta portion of the acetic acid stream in a drying column to form a waterstream and an acetic acid product stream; (c) separating the PRCenriched stream into an aqueous phase and an organic phase; (d)separating the aqueous phase in a distillation column into an overheadstream and a bottoms stream; (e) extracting at least a portion of theoverhead stream with an extraction medium to form a PRC extracted streamand a raffinate; (f) directing a first return stream comprising analiquot portion of the bottoms stream to the light ends column; (g)directing at least a portion of the raffinate to the light ends column,optionally as part of the first return stream; and (h) directing asecond return stream comprising an aliquot portion of the bottoms streamto the drying column.